In recent years, the use of the ferroelectric materials for random access memory (RAM) elements has reached commercial applications in the semiconductor industry. Ferroelectric dielectric materials have very high dielectric constants (typically .epsilon.&gt;500) providing for memory elements that have high charge storage capacity. Ferroelectric memory elements are non-volatile, consume low power and are programmable with low voltage, e.g. less than 5V. Other advantages include fast access times, (&lt;40 ns), radiation hardness, and robustness with respect to virtually unlimited read and write cycles.
Ferroelectric dielectrics are of also of interest for applications as coupling and decoupling capacitors, and for filter elements operating at low frequency (&lt;1 Hz) up to microwave (GHz) frequencies. The relatively low value of the dielectric constant of conventional dielectrics, typically silicon dioxide and silicon nitride (.epsilon.&lt;10) limits the capacitance attainable to about 2 to 3 fF/.mu.m.sup.2. The high dielectric constant of ferroelectric dielectric materials allows for capacitances greater than 30 fF/.mu.m.sup.2. A number of integrated circuit applications would benefit from large on-chip capacitances in the nF range. Consequently, there is much interest in ferroelectric dielectric materials for larger, high value capacitors, as well as for smaller memory elements.
Since ferroelectric materials may also exhibit useful piezoelectric and non-linear optical properties, there is also much interest in providing improved methods for making thin film ferroelectric materials for optoelectronics and other applications, e.g. waveguides, electro-acoustic transducers, surface acoustic wave devices, non-linear optical devices, optical modulators and piezoelectric devices. The high piezoelectric coupling coefficient of ferroelectric materials make them suitable as actuators for micromachine structures incorporated in silicon micro-circuits.
Ferroelectric dielectric materials with large dielectric constants include ferroelectric perovskites, which are complex metal oxides of the general structure ABO.sub.3 in which the A and B sites of the perovskite structure are occupied by one or more different metals. Particular perovskite ferroelectric materials which have made the breakthrough in integrated circuit applications include, for example, lead zirconate titanate PbZr.sub.x Ti.sub.1-x O.sub.3 (PZT), lead lanthanum zirconium titanate (PLZT), barium titanate (BT), and barium strontium titanate (BST).
PZT has a higher dielectric constant and may be formed at lower temperature than BST. On the other hand, PZT formed by conventional methods shows dispersion, at frequencies above .about.100 MHz, above which the dielectric constant drops to a low value. BST has a flat dielectric response up to .about.5 GHz and is favoured for high frequency GaAs integrated circuit applications.
The interest in using ferroelectric materials for applications in non-volatile DRAMs has led to rapid development of improved processes for deposition of thin layers of ferroelectric dielectric materials. Known deposition methods which have been investigated include, for example, metallo-organic sol-gel processes, and other spin-on liquid processes based on metallo-organic decomposition, chemical vapour deposition (CVD) and sputtering, laser ablation, electron beam deposition and ion beam deposition.
The integration of ferroelectric materials for capacitor dielectrics for integrated circuits, or for other device structures, requires a process which is compatible with known semiconductor process technologies. Furthermore, the properties of ferroelectric materials provided as thin films are found to differ from bulk ferroelectric materials. In comparison with preparation of bulk ferroelectric materials, factors including film stress, interactions with substrate materials, and restrictions on process temperatures may significantly influence the characteristics of thin films of ferroelectric materials. Thus, much work has been devoted to developing low temperature processes for formation of thin films of ferroelectric dielectrics compatible with semiconductor processing for CMOS, bipolar and bipolar CMOS technologies.
For integrated circuit applications, a preferred known process for forming thin films of ferroelectric materials is based on a technique, generally known as a sol-gel process, in which a complex oxide is prepared from a sol-gel precursor solution comprising a mixture of metallo-organic.sup.1 compounds, e.g. alkoxides dissolved in an organic solvent, and/or organic metal salts dissolved in an appropriate solvent e.g. an acid or alcohol. The sol-gel process for preparing metal oxides proceeds by the hydrolysis of a metal organic compound to form a sol comprising metal oxide precursors. This process is well known for forming single component oxide glasses and multi-component oxide glasses from a precursor mixture of metal alkoxides. Formation of metal oxide bonds and growth of metal oxide chains and networks in the solution eventually lead to gelation. The hydrolysis and polymerization by condensation (polycondensation) reactions are controlled by factors such as the amount of water, pH, presence of acid or base catalysts, and reaction sequence, for example, as described in U.S. Statutory invention registration no. H 626, published Apr. 4, 1989, entitled "Sol-Gel Ceramic Oxides" to Covino which relates to formation of silicate glasses. In the latter disclosure, it is described how it is known that lowering of pH tends to form oxide networks and chains, forming a polymer network, and leading to gelation without formation of colloidal oxides. FNT .sup.1 In the context of sol-gel processing of complex oxide ceramics, the term organo-metallic or metallo-organic has often used to denote metal containing organic compounds used as precursors, including metal alkoxides, metal carboxylates and metal beta diketonates. In organic chemistry, the term "organo-metallic" or metallo-organic is more generally used to denote a compound having a metal-carbon bond.
In a conventional method of sol-gel processing of piezo-electric and ferroelectric dielectric thin films of the general formula ABO.sub.3 using a sol-gel precursor solution, a precursor solution is provided comprising metal A as an organo-metallic salt, e.g. a metal acetate, and a mixture of metals B as alkoxides, provided in the required stoichiometric proportions. For example, to make PZT, a precursor mixture of a soluble organic lead salt, e.g. lead acetate tri-hydrate, and a mixture of zirconium propoxide Zr(OC.sub.3 H.sub.7) and a titanium iso-propoxide Ti(OC.sub.3 H.sub.7).sub.4 is dissolved in a suitable solvent e.g. an alcohol, or mixture of solvents. The lead salt is dissolved in a suitable anhydrous solvent such as methoxy-ethanol, and the solution is dehydrated, and then the zirconium and titanium propoxides, also dissolved methoxy-ethanol, are added in stoichiometric ratio to provide the B metal cations.
The metal oxide precursor solution forms a mixture of metallo-organic intermediate compounds which react to form a metal oxide precursor. The viscosity and surface tension of the precursor solution is adjusted to allow a layer with a controlled thickness to be spin-coated or dip-coated onto a substrate, as required, depending on the particular application.
Organic metal oxide precursors other than metal alkoxides which have been reported include metal beta-diketonate (e.g. acetyl acetonate) or metal carboxylates, e.g. acetates. For example U.S. Pat. No. 4,946,710 to Miller entitled "Method for preparing PLZT,PZT,and PLT sol-gels and fabricating ferroelectric thin films", lists precursors including lead acetate, lead tetraethylhexanoate, zirconium acetyl acetonate, titanium tetra-butoxide butanol complex, titanium iso-propoxide, zirconium tetra-butoxide, lanthanum 2,4, pentadionate, and other acetates and alkoxides which are commercially available and form organo-metallic polymer gels. Nevertheless it is noted that precursor compounds with bulky organic groups are likely to result in porous materials with defects, and thus metal compounds derived from methanol, ethanol, butanol, propanol and acetic acid derived compounds were preferred
Desirably, there is a common solvent for all precursor solutions of metals A and B. When different solvents are used, solubilization of the metal precursors in different solvents may hinder the formulation of a homogeneous sol-gel solution. In known methods, solvents comprising alcohols, methanol, butanol, propanol, etc. and methoxy-alcohols, e.g. methoxy-ethanol, 1-methoxy-2-propanol are preferred.
In PZT, the ratio of zirconium to titanium occupying the B sites of the ABO.sub.3 structure may be varied, and is typically in the range from 20:80 to 80:20, with 40:60 ratio of Zr:Ti being typical. In doped or modified materials, another metal may occupy a proportion of the A sites. One particular example is lead lanthanum zirconium titanate (PLZT) where some of the A sites are filled by lanthanum. PZT may alternatively be modified or doped with other metals, including niobium, tantalum, iron, aluminum and others. These metals are also added in the desired proportion to the sol-gel precursor solution in the form of organo-metallic compounds, typically as metal alkoxides M(OR).sub.x or metal carboxylates, typically a metal acetate. Dopant metals are alternatively added as chlorides of iron, or vanadium, e.g. VCl.sub.3, or a chloride or nitrate of lanthanum.
Conventionally, to provide a thin layer of ferroelectric, a substrate is spin-coated or dipped to provide a thin layer on the substrate, and then a heating step at relatively low temperature, .about.200.degree. C. to 400.degree. C., results in pyrolysis, i.e. thermal decomposition of the organo-metallic.sup.1 compounds, and drives off solvent and volatile organics, and leaving a layer comprising the mixed metal oxides, which transform at sufficiently high temperature, to form a layer of the ferroelectric mixed oxide. The lower temperature step is often referred to as "firing", or "baking". Heating to a higher temperature, typically 600-800.degree. C., causes crystallization to a ferroelectric perovskite phase of a mixed oxide having the required functional properties. The higher temperature step may is referred to variously, e.g. as annealing, or sintering. It is known that the nature of the ferroelectric film is sensitive to the substrate, and to the processing conditions.
Bernstein et al. in European Patent Application No. 0 489 519 A2, entitled "Sol-gel processing of piezoelectric and ferroelectric films", report that rapid localized heating avoids formation of a crust on the surface of the layer, which may prevent outgassing of volatile organic components during heating in a conventional furnace tube. Rapid heating, e.g. by localized heating the substrate on a hot plate, was found to cause localized stabilization of the film while allowing continued outgassing of organics from the surface, a technique that was found to be advantageous in lowering the crystallization temperature. It was also reported that properties of film were more linear with applied voltage when crystallization was carried out in an inert or reducing atmosphere rather than an oxidizing atmosphere.
Bernstein refers to the method of Sayer and Yi described in International Patent Application WO-90/13149, entitled "Sol gel process for preparing Pb(Zr,Ti)O.sub.3 thin films". Sayer used acetic acid as a solvent for both lead acetate trihydrate and zirconium and titanium propoxides, rather than methoxy-ethanol, which is a known teratogen. Sayer and Yi provide a method of preparing crack free thin films by addition of what is called a "firing additive", e.g. a glycol such as, ethylene glycol, glycerol, tetra-ethylene glycol, or a polyethylene glycol, for adjusting the viscous state transition temperature of the sol-gel precursor solution. After heating at 300.degree. C. to 500.degree. C. to pyrolyse the film, crystallization was induced by a lengthy 6 hour anneal at about 600.degree. C. However, for integrated circuit applications, a more rapid anneal step is desirable.
In the earlier work of Sayer et al. described in the above mentioned EP Patent Application, the choice of precursor compounds and solvents is important in controlling the characteristics of thin film ferroelectric materials. Precursor compounds should preferably have a high metal content, high solubility in the selected solvent, decompose without evaporating, and be chemically compatible with one another. The solvent must have appropriate boiling point and suitable viscosity and surface tension. Water and/or propanol may be added to adjust the viscosity, and reduce surface tension to increase wettability of the substrate. A chelating agent is added to prevent hydrolysis of the sol-gel solution, and is preferably glacial acetic acid, although other acids may be used.
The order of mixing of the precursor constituents may also be important. For example, Sayer reported that in mixing the lead salt and metal alkoxides, it is important to add zirconium propoxide first because it reacts with acetic acid to form a non hydrolyzable solution which protects the titanium iso-propoxide from hydrolysis. If Ti iso-propoxide is added first, it reacts with the acetic acid to form mono- or di-acetylates and condensation occurs with the formation of poly-titanyl acetylates. In later work discussed in an article entitled "Sol-gel processing of complex oxide films", in Ceramic Bulletin, vol. 70, no. 7, 1991, pages 1173-1179, Yi and Sayer mixed the Ti and Zr precursors before addition of the lead salt and acetic acid. This procedure has been followed by Schwartz et al. under the name "inverted mixing orders" as reported in Integrated Ferroelectrics vol. 2, 1992, pages 243-254, entitled "Solution chemistry effects in Pb(Zr,Ti)O.sub.3 thin film processing".
To provide a homogeneous sol-gel solution, the mixture is agitated, preferably in an ultrasonic tank, until all solids are dissolved. A filtered solution is stable in air, and may be stored in a sealed container.
Sayer et al. also highlight that the low temperature heating step, which they term "firing", which pyrolyses the organo-metallic compound to an inorganic film, is key to the preparation of the crack free films having desired characteristics including crystal structure, grain size, transparency, and surface roughness.
Evaporation of the solvents and volatile components causes a large volume change, and thus generates internal stresses. During firing, volatile organic components are driven off and the organic film changes to fine mixture of metal oxides, and free carbon. Then at higher temperature the free carbon oxidizes, is released as carbon dioxide, and the mixture of oxides transforms to a transparent amorphous PZT film. Processing and firing under vacuum was found to be advantageous in extracting water uniformly and for uniform decarbonization.
In the method of Sayer et al., it is believed that high boiling point and latent heat of a glycol additive raises the solution evaporation temperature in the first stage towards the melt temperature in the second stage, which retains atom mobility and reduces tendency for cracking.
Other sol-gel based processes for providing improved quality ferroelectric thin films are described in the following references.
Swartz et al. in U.S. Pat. No. 5,198,269 entitled "Process for making sol-gel deposited ferroelectric thin films insensitive to their substrates", describe a stepwise process for improving the quality of a perovskite ferroelectric thin film by depositing a thin "interlayer", of a first perovskite material, e.g. PbTiO.sub.3, or SrTiO.sub.3 on which is adherent to the substrate, before depositing a second perovskite material, e.g. PLZT or other ferroelectric material. The interlayer was found to improve crystallinity of the PLZT and provide for deposition on a wider range of substrates.
Maniar in U.S. Pat. No. 5,271,955 and continuation U.S. Pat. No. 5,391,393, both entitled "Method for making a semiconductor device having an anhydrous ferroeelctric thin film", describes a sol-gel method in which an anhydrous sol-gel precursor is prepared, without hydrolyzing the sol-gel solution. By using anhydrous lead acetate, there is no need to dehydrate the solution, and this compound shows enhanced reactivity with other components. Unlike other known methods in which a condensate is formed by mixing the solutions and then hydrolyzing, this method uses thermally induced condensation, i.e. by boiling (or refluxing) to induce a heterogeneous condensation reaction between metal precursors in solution. Preparation of the anhydrous solution in an oxygen containing dry ambient resulted in a more stable mixture, reduced degradation of the solution by atmospheric humidity and increased shelf life. Maniar notes that exclusion of water avoids bulky precursor molecules with a high degree of internal strain which occurs with hydrolyzed precursors that tend to polymerize preferentially with a single metal element. Films were prepared from the anhydrous sol-gel solution in a conventional manner, with a heating step to drive off solvent and organic ligands, followed by sintering to interdiffuse metals and form perovskite thin film.
A long shelf life of anhydrous sol-gel solution was reported, using excess lead from 0 to 20%. Excess lead is known to suppress formation of an intermediate pyrochlore phase during annealing. The formulation of the sol-gel exclusively by thermal condensation and in the absence of hydrolysis yields an anhydrous amorphous sol-gel having a uniform condensate composition. The method provides improved durability and lower temperature conversion to perovskite crystalline phase.
Mackenzie et al. in U.S. Pat. No. 5,342,648, entitled "Method for forming amorphous ferroelectric materials" discusses how the morphology of polycrystalline thin films dictates characteristics of the material, and suggests growing single crystal films by sol-gel techniques to avoid shortcomings introduced by grain boundaries in polycrystalline films. Mackenzie uses a precursor mixture of metal alkoxides dissolved in alcohol such as absolute ethanol to provide an anhydrous mixture which is not reacted with water until it is coated onto the substrate. PZT is prepared from a mixture of Ti propoxide and Zr propoxide dissolved in propanol, mixed with lead acetate dissolved in propanol. Hydrolysis and polycondensation of this mixture occurs in situ in the thin film and provides amorphous thin films. That is, when water, e.g. water vapour in air reacts with the thin film, and by control of humidity during processing, a polycondensation of a polymer having metal oxygen-metal bonds occurs controllably. The alkyl groups are released as corresponding alcohol. A pre-polymer may occur as particles in the gel.
Teeowee et al., in U.S. Pat. No. 5,384,294 entitled "Sol-gel derived lead oxide containing ceramics" report a method for production of PZT using a mixture of a lead carboxylate (e.g. lead acetate) in alcohol, and other metal cations provided as a mixture of alkoxides and alkanolamines, i.e. amine derivatives of the more commonly used alkoxides. These are prepared by reacting metal alkoxides with an amine. The alkanolamines are less reactive and less hygroscopic with improved solubility in higher alcohols. Shelf life is prolonged relative to unmodified metal alkoxides. PVP (polyvinyl pyrollidine) is added for sol rheology control, i.e. to control the viscosity and flow properties of the sol-gel solution. Thin films with exceptionally high dielectric constants, up to 3000, were obtained.
A number of processing difficulties arise in known processes including lack of batch to batch uniformity and reproducibility due to instability and degradation of the sol-gel precursor solutions. Special precautions are required for making and storing anhydrous solutions to keep out atmospheric moisture.
Non-uniformities in coating may occur due to inadequate control of the viscosity or surface tension of organic solutions. Cracking may occur during the heating phase, as a result of stress and macroscopic defects generated by the large volume changes when volatile organic components and solvents are driven off. An excessive volume ratio of organic products to the inorganic polymer network can cause porosity and cracks in the fired films, may inhibit reaction of the precursor film components, and give rise to poor crystallization in the fired film. Stress, or poor adhesion to the substrate then may result in delamination of films. Stress and cracking are exacerbated in thicker films (&gt;1 .mu.m).
While it has been reported that cracking of films may be controlled by firing at lower temperature for extended periods, (i.e. initial heating stage in excess of 20 minutes), extended thermal processing may not be compatible with integrated circuit fabrication, or may result in low quality crystalline layers, with poor ferroelectric characteristics. Rapid thermal processing is preferable for integrated circuit fabrication. However, rapidity of reaction may exacerbate any inhomogeneities in the film, and generation of stresses are significantly different in rapid thermal processing as compared with furnace annealing.
As in all semiconductor processing, high purity starting reactants are desirable. Nevertheless, because organic metal precursors are used, carbon containing residues of the volatile organic precursors may remain in the film, either in the bulk, or trapped at boundaries between grains. Oxygen loss from the structure may occur during oxidation of residual carbon to carbon dioxide, which results in an oxygen deficient stoichiometry of the ferroelectric phase.
Oxygen stoichiometry is important because the functional properties of the ferroelectric oxide are strongly dependent on the "oxygen octahedra" in the crystal structure. A minimum oxygen stoichiometry is required to maintain the non-centro symmetry of the unit cell. Under most circumstances, known chemical processes yield an oxygen stoichiometry above the critical limit to form a desired crystal structure, and the film is identifiable as a ferroelectric. Nevertheless, oxygen non-stoichiometry, remains a major difficulty to be overcome. If oxygen stoichiometry occurs non-uniformly throughout the film, the average properties of the material will be degraded. For example, processed ferroelectric films generally show dielectric constants which are lower and coercive fields for polarization reversal which are higher than those observed in the bulk material. Films are often not able to withstand repeated polarization reversals for as many repetitions as required. Thus, removal of carbon from the as-coated films is identified as a critical processing stage, and known methods suggest that removal of carbon is best achieved by firing (heating) the films at 350.degree. to 400.degree. C. for a few minutes. Apart from organic precursors, and organic solvents, other organics added as firing additives or to adjust surface tension and viscosity also add to the organic loading of the solution. In lead containing ferroelectrics, the organic lead salt is usually added in excess to suppress formation of a pyrochlore phase. Thus, each of these components contributes to an excess of carbon, and thus may adversely affect oxygen stoichiometry.